The present invention relates to a process for the deposition of protective coatings on Si-based substrates used in articles subjected to high temperature, aqueous environments, and the resulting article.
Ceramic materials containing silicon have been proposed for structures used in high temperature applications, for example, gas turbine engines, heat exchangers, internal combustion engines, and the like. A particular useful application for these materials is for use in gas turbine engines which operate high temperatures in aqueous environments. It has been found that these silicon containing substrates can recede and lose mass as a result of a formation volatile Si species, particularly Si(OH)x and SiO when exposed to high temperature, aqueous environments. For example, silicon carbide when exposed to a lean fuel environment of approximately 1 ATM pressure of water vapor at 1200° C. will exhibit weight loss and recession at a rate of approximately 6 mils per 1000 hrs. It is believed that the process involves oxidation of the silicon carbide to form silica on the surface of the silicon carbide followed by reaction of the silica with steam to form volatile species of silicon such as Si(OH)x.
Suitable coatings for articles containing silicon based substrates which are employed in the environments claimed above are well known in the art. See for example U.S. Pat. Nos. 5,305,726; 5,869,146; 6,284,325; 6,296,941; 6,352,790; and 6,387,456. The environmental barrier coatings (EBCs) are generally based on a two or three layer design utilizing a bond coat, an optional intermediate layer, and an environmental protection layer. The bond coat may be, for example, a dense continuous layer of pure silicon or silicon with a modifier. The top protective layer may be an alkaline earth aluminosilicate based on barium and/or strontium or a simple silicate based system such as yttrium silicate either of which are chosen in part by matching the coefficient of thermal expansion (CTE) of the top layer to the under layer and/or substrate. Other top layer systems may also be used. An intermediate layer, if employed, may be for example, a mixture of the top layer with a second phase that in combination serves to provide a barrier action and/or to help modify the CTE of the system. Engine testing to date of EBC designs in combustor locations has shown significant benefit for industrial gas turbines use at temperatures of up to 1200° C. and for durations of more than 15,000 hrs.
Use of these EBC designs as described above, at advanced surface temperatures of up to 1500° C., especially under thermal gradient conditions, have been shown to suffer from changes in thermal resistance of the EBC coating. The change is due to at least three phenomenon. First, changes in microstructure of the EBC can result due to high temperature exposure. As-fabricated, thermal sprayed structures have a splat quenched, layered, non-equilibrium microstructure. On high temperature exposure the structure can equilibrate losing the splat-quenched microstructure. This loss alters the light scattering behavior of the structure and this in turn affects radiant energy transmission through the coating. Second, because of densification of the microstructure, the thermal conductivity of the coating increases. Third, at a higher temperature, the peak wavelength of emitted light radiation occurs at shorter wavelength. This phenomenon can result in effective reduction in thermal resistance of the EBC if the EBC is more transparent to radiant energy at shorter wavelength. Generally, these phenomena are observed at temperatures of 1200° C. to 1500° C. for laboratory time scales.
Naturally, it would be highly desirable to provide improved top layers for EBC's, which are thermally stable at temperatures up to at least 1500° C.
Accordingly, it is a principle object of the present invention to modify heretofore known top layers for EBC's so as to extend the useful life of the top layer in temperature environments of up to at least 1500° C.
It is a further object of the present invention to provide a top layer as aforesaid which includes additives which stabilize as fabricated properties and/or affect the mechanisms of thermal resistance based on transmission of radiant energy in connection with top layers.